The present invention relates to an apparatus and a method for controlling fuel injection in an internal combustion engine that includes an injection valve for injecting fuel into a cylinder and an injection valve for injecting fuel into an intake passage. Particularly, the present invention relates to a technique for preventing a component provided in an exhaust passage, such as catalyst for purifying exhaust gas, from being overheated.
Japanese Laid-Open Patent Publications No. 5-231221, No 7-103050, and No. 2001-20837 each disclose an internal combustion engine having an injection valve for directly injecting fuel into a cylinder (in-cylinder injection valve) and an injection valve for injecting fuel into an intake passage (intake cylinder injection valve). In such an internal combustion engine, various fuel injection modes can be performed using the two types of injection valves, thereby permitting the engine to be finely controlled according the engine operational state. For example, when the intake passage injection valve is used for injecting fuel, homogeneous air-fuel mixture is formed in the cylinder and such mixture is combusted (homogeneous combustion).
On the other hand, when the in-cylinder injection valve is used for injecting fuel, the engine is operated in a combustion mode selected from a stratified combustion mode and a homogeneous combustion mode. In the stratified combustion mode, fuel is injected from the in-cylinder injection valve during the compression stroke of the engine so that relatively rich air-fuel mixture is formed in the vicinity of the ignition plug. In this state, the mixture is ignited to perform combustion. The stratified combustion mode permits the engine to operate by the combustion a relatively lean air-fuel mixture. Accordingly, the fuel economy is improved and the emission of CO2 is reduced.
In the homogenous combustion mode, fuel is injected from the in-cylinder injection valve during the intake stroke of the engine so that homogenous air-fuel mixture is formed in the cylinder. In this state, the mixture is ignited to perform combustion. Air that is drawn into the cylinder is cooled by the effect of heat of evaporation of injected fuel. Accordingly, the filling efficiency of air to the cylinder is increased. Therefore, when the engine is operated in the homogeneous combustion mode, the engine produces greater power.
Also, a fuel injection mode in which fuel is injected from both of the in-cylinder injection valve and the intake passage injection valve can be performed.
A catalyst for purifying exhaust gas is provided in the exhaust system of an engine. If the catalyst is overheated due to a temperature increase of exhaust gas, the purifying performance of the catalyst can be degraded, and the life of the catalyst can be shortened. Japanese Laid-Open Patent Publication No. 2002-130011 discloses a technique in which the amount of fuel supplied to a cylinder is increased to prevent a catalyst from being overheated. However, the technique disclosed in the publication is only applicable to engines having an in-cylinder injection valve, but cannot be favorably applied to engines having an in-cylinder injection valve and an intake passage injection valve.
Japanese Laid-Open Patent Publication No. 7-103050 discloses a technique for switching injection valves. Specifically, when an abnormality in fuel injection from an in-cylinder injection valve is detected, fuel injection from the in-cylinder injection valve is stopped and fuel injection from an intake passage injection valve is started. An abnormality of fuel injection degrades the combustion state and affects the temperature of exhaust gas, in other words, affects the temperature of a catalyst. However, Japanese Laid-Open Patent Publication No. 7-103050 only discloses that the injection valve to inject fuel is switched to the intake passage injection valve when an abnormality is detected in fuel injection from the in-cylinder injection valve. In the publication, overtemperature of the catalyst is not taken into consideration at all.